Japanese Laid-open Patent Publication No. 2011-141478 has disclosed a structure of a multi-channel optical transmitter in which a signal light condensed by a lens is branched, and is monitored by a monitoring PD for each channel.
However, in the technique in Japanese Laid-open Patent Publication No. 2011-141478, a light emitting element and a light receiving element are provided with a same substrate, and thus the output wire lines of the light receiving element have to be provided with that substrate. In this case, the thickness of an optical transmitter increases, and the device becomes large in size.